Exposed photothermographic media (film, paper) is developed by a thermal processor. A typical thermal processor includes a heated drum to which the media is held by means of a belt or rollers, such as disclosed in U.S. Pat. No. 4,304,985, issued Dec. 8, 1981, inventor Miller; and PCT unexamined International Application WO 95/30934, published Nov. 16, 1995, inventors Star et al. In the latter application, the hold down rollers are of hollow or solid metallic material that are thermally conductive, as a result of exposed film in passing over the drum surface has a tendency to sink heat from the rollers at an accelerated rate for the first several films in a burst. The consequence of this is that the roller temperatures drop, thereby giving less heat energy to the film until an equilibrium is reached. This results in a density shift from film to film during burst. The temperatures of the pressure rollers drift depending on the time sequence of sheets being processed. The amount of drift is bounded by a maximum equal to the drum temperature setpoint, T0 and a minimum equal to the temperature at which zero roller-to-film heat transfer occurs (lowest for the first roller and increasing for succeeding rollers). As roller temperatures drift, corresponding variations in roller-to-film heat transfer occurs. If a simple thermal control servo is used to hold the drum temperature perfectly constant at some fixed sensor location, then these variations in roller-to-film heat transfer will cause corresponding variations in the time-temperature profile of the emulsion. The end result of all this is decreasing sheet-to-sheet density (a repeatability problem)--even though the drum temperature is held perfectly constant at the sensor location. It has been found that the density can decrease by as much as 10-15% for successive processing of ten or more sheets.
There exists a need therefore for a thermal control process that compensates for roller temperature variations.